Proteinases are present in both prokaryotic and eukaryotic systems and have been shown to play an important role in the processing of large precursor polyproteins during viral replication. The reliance of viruses on proteolytic processing by virally encoded proteases has been suggested to have several evolutionary advantages, such as a need for reduced genomic content and partial release from the constraints of the mechanisms of transcriptional and translational regulation of the host cell (Lawson et al., 1990). Viruses have evolved methods for regulating the proteolytic cascade that produces viral structural and replication proteins to replace the host mechanisms. Lawson et al. (1991) report that in picornaviruses, the temporal and spatial distribution of expressed protease activity affects the appearance and location of the final proteolytic product.
Potyviruses are members of the picornaviral superfamily that infect plants. Like other members of the picornaviral family, potyviruses make extensive use of proteinases during replication. An example of a potyvirus that possesses a genome encoding a single large polyprotein proteolytically processed by virally encoded protease is the tobacco etch virus (TEV). TEV has a single-strand, plus-sense RNA genome of about 9,500 nucleotides. The RNA is organized as a single open reading frame and encodes a 346 kDa polyprotein (Allison et al., 1986). The polyprotein is co- and post-translationally processed by viral encoded proteinases. Two proteinases, P1 and helper component proteinase (HC-Pro), are responsible for their autocatalytic release from the amino-terminus of the polyprotein (Verchot et al. 1991; Carrington et al., 1989). The third proteinase, nuclear inclusion proteinase (NIa), mediates all other cleavage events.
Characterization of the 27 kDa NIa Proteinase of Potyvirus
The NIa proteinase is 49 kDa and is found as an aggregate with the 54 kDa NIb polypeptide in nuclear inclusion bodies in infected plant cells (Carrington et al., 1988; Parks et al., 1995). The 49 kDa NIa proteinase is a picornavirus 3C-like proteinase that recognizes cleavage sites within the C-terminal two-thirds of the polyprotein. The proteolytic domain of NIa lies within the C-terminal half of the protein and has a molecular weight of about 27 kDa, while the N-terminal region of NIa contains the Vpg (viral protein, genome-linked) activity and has a molecular weight of about 21 kDa.
Structurally, the 27 kDa NIa proteinase has been reported to be similar to the trypsin-like family of cellular serine proteinases, such as chymotrypsin or trypsin, with the substitution of Cys for serine as the active site nucleophile (Blazan et al., 1990; Dougherty et al., 1989). Dougherty et al. (1989) disclose that the catalytic triad of 27 kDa proteinase is composed of His, Asp, and Cys, being similar to the catalytic triad found in other viral proteinases (Dougherty et al., 1989). However, unlike the other proteinases, the 27 kDa proteinase recognizes an extended heptapeptide sequence, E-X-X-Y-X-Q↓S/G (positions P6-P1↓P′1; X is any amino acid) (SEQ ID NO: 1), and cleaves within the heptapeptide sequence (Dougherty et al., 1989a; Dougherty et al., 1988; Dougherty et al., 1989b). Residues at positions P6, P3, P1, and P′1 are conserved and essential for optimal cleavage. Amino acids at the other positions appear to modulate the rate at which cleavage occurs (Dougherty et al., 1989; Dougherty and Parks, 1989).
Moreover, the 27 kDa NIa proteinase appears to be structurally and functionally similar to other plus-stranded RNA viral-encoded proteinases (Krausslich and Wimmer, 1998). First of all, it cleaves the polyprotein between particular Gln-Gly or Glyn-Ser dipeptides. Secondly, proteolytic activity is enhanced by dithiothreitol. Thirdly, the gene encoding this proteinase is adjacent to the putative RNA-dependent, RNA-polymerase gene. Lastly, the proteinase contains a conserved C-terminal amino acid motif (Cys-˜15 amino acids-His) (Argos et al., 1984). This last characteristic is shared by proteinases encoded by many RNA viruses that translationally express their genetic information as a single polyprotein from genome length RNA (Dougherty et al., 1989).
Additionally, Parks et al (1995) report that the 27 kDa NIa proteinase contains an internal self-cleavage site positioned at 24 amino acids from the carboxyl terminus of the proteinase and that the active 27 kDa proteinase converts to a lower molecular weight form with time. The 27 kDa NIa proteinase lacking the C-terminal 24 amino acids exhibits limited activity. The truncated proteinase is about one-twentieth as efficient in proteolysis of a test peptide substrate as the full length form, and Parks et al. (1995) indicate that the 27 kDa NIa proteinase appears to lose its activity with time.
Further, Polayes et al. (1994) disclose that the 27 kDa NIa proteinase is a highly specific protease that is active under a broad temperature range and on a variety of substrates. Polayes et al. report rapid cleavage at 30° C. and 37° C., about 80% cleavage at both 21° C. and 16° C. in one hour, and 50% cleavage at 4° C. Accordingly, Polayes et al. recommend the use of this proteinase as a tool for removing affinity tags from fusion proteins.
Use of 27 kDa NIa Proteinase Cleavage System for Purification of Proteins
Parks et al. (1994) disclose an improved method for the production, cleavage, and purification of fusion proteins and peptides using the 27 kDa NIa proteinase. The method comprises producing a fusion protein comprising the protein of interest, a carrier peptide (such as an affinity carrier) and a 27 kDa NIa proteinase cleavage site inserted between the two, purifying the fusion protein, and incubating the fusion protein with the 27 kDa Nia proteinase to remove the carrier peptide from the protein of interest.
Johnston et al., U.S. Pat. No. 5,532,142, disclose a similar method of isolation and purification of recombinant proteins using the 27 kDa NIa proteinase. Like the purification method of Parks et al. (1994), the method of Johnston et al. involves producing large quantities of the fusion protein containing a desired protein fused to the 27 kDa NIa proteinase cleavage site which is the carrier peptide, purifying the fusion protein, and incubating the purified fusion protein with the 27 kDa NIa proteinase to remove the carrier peptide from the desired protein.
Unlike other proteinases, the 27 kDa proteinase exhibits high specificity, insensitivity to many proteinase inhibitors used in protein purification, and efficient cleavage under a broad range of temperatures (Polayes et al., 1994). Moreover, the protein of interest is easily separated from the carrier peptide and the 27 kDa proteinase. For these reasons, there is an on-going interest in obtaining large quantities of the active protein for use as a tool in protein purification.
Purification of Proteins Including the 27 kDa NIa Proteinase from Inclusion Bodies
Purification of Proteins that Form Inclusion Bodies. The development of recombinant DNA technology has enabled the cloning and expression of proteins in bacteria, yeast and mammalian cells and has made it possible to produce therapeutics and industrially important proteins at economically feasible levels. However, the expression of high levels of recombinant proteins in Escherichia coli often results in the formation of inactive, denatured protein that accumulates in intracellular aggregates known as insoluble inclusion bodies (Krueger et al., “Inclusion bodies from proteins produced at high levels in Escherichia coli,” in Protein Folding, L. M. Gierasch and P. King (Eds), Am. Ass. Adv. Sci., 136-142 (1990); Marston, Biochem. J. 240:1-12 (1986); Mitraki et al., Bio/Technol. 7: 800-807 (1989); Schein, Bio/Technol. 7:1141-1147 (1989); Taylor et al., Bio/Technol. 4: 553-557 (1986)). Inclusion bodies are dense aggregates, which are 2-3 m in diameter and largely composed of recombinant protein, that can be separated from soluble bacterial proteins by low-speed centrifugation after cell lysis (Schoner et al., Biotechnology 3:151-154 (1985)).
The recovery of recombinantly expressed protein in the form of inclusion bodies has presented a number of problems. First, although the inclusion bodies contain a large percentage of the recombinantly produced protein, additional contaminating proteins must be removed in order to isolate the protein of interest. Second, the proteins localized in inclusion bodies are in a form that is not biologically active, presumably due to incorrect folding.
Several methods have been developed to obtain active proteins from inclusion bodies. These strategies include the separation and purification of inclusion bodies from other cellular components, solubilization and reduction of the insoluble material, purification of solubilized proteins and ultimately renaturation of the proteins and generation of native disulfide bonds. The art teaches that concentrations of 6 M or greater of chaotropic agents, such as guanidine hydrochloride, guanidine isothiocyanate or urea-are necessary for solubilization of the insoluble recombinant polypeptides from the inclusion bodies. See, for example, Vandenbroeck et al, Eur. J. Biochem. 215:481-486 (1993); Meagher et al., Biotech. Bioeng. 43:969-977 (1994); Yang et al., U.S. Pat. No. 4,705,848, issued Nov. 10, 1987; Weir et al., Biochem. J. 245:85-91 (1987); and Fischer, Biotech. Adv. 12:89-101 (1994). However, the use of high concentration of chaotropic agents, such as guanidine hydrochloride, to solubilize proteins denatures the proteins.
U.S. Pat. No. 5,912,327 discloses the use of low concentrations of guanidine salts, about 0.7 to about 3.5 M, to solubilize biologically active (i.e., correctly folded) proteins and extract this population of the protein from a heterogenous protein mixture localized in inclusion bodies. The method described in the patent comprises releasing the inclusion bodies containing the target protein from the cells by lysis, optionally washing the cells to remove cellular components, extracting with solutions containing low concentrations of guanidine salts, refolding target proteins which have been solubilized using guanidine salts by rapid dilution of guanidine salt extracts and optionally employing agents which facilitate target protein refolding. The protein can then be recovered and purified by methods well known to the skilled artisan. However, this method is labor intensive.
Tissue plasminogen activator (tPA or TPA) is one example of a pharmaceutically important drug produced by recombinant methods. Unfortunately the current methods for producing tPA from bacterial cell culture are both costly and laborious. The production of tPA in heterologous host organisms relies on the production of inactive tPA intracellularly in inclusion bodies, and the subsequent isolation and purification of such inclusion bodies, followed by activation of the tPA once freed from the inclusion bodies. U.S. Pat. No. 5,077,392 discloses a renaturation method for refolding denatured proteins obtained after expression in inclusion bodies. tPA is isolated as a denatured reduced protein and on subsequent oxidation refolded under oxidizing conditions to obtain what was reported as up to a 26% yield of “reactivated” protein. While the method appeared to improve polypeptide yield, the process involves multiple, time-consuming steps, due to the initial recovery of the insoluble, inactive protein.
Purification of 27 kDa NIa Proteinase. The 27 kDa NIA proteinase has been especially difficult to isolate and purify in large quantities and in active form because of its proclivity to form inclusion bodies in nature. Previously published purification protocols of TEV nuclear inclusion bodies from infected plant tissue have demonstrated considerable proteolytic activity (Dougherty et al., 1980). However, attempts to separate the 49 kDa NIa proteinase from the Nib protein and other components and to purify the NIa proteinase have resulted in loss of protein activity (Parks et al., 1995).
Parks et al. (1995) describe purification of the soluble fraction of recombinantly produced 27 kDa NIa proteinase. The purification method of Parks et al. involves overexpressing the recombinant form of the proteinase as a fusion protein comprising a seven-His tag at the N-terminus and purifying the fusion protein using two separate columns, a nickel-nitrilotriacetic acid-agarose (Ni-agarose) column and a cation-exchange column. This method is labor-intensive and produces insufficient quantities of proteinase for use as a general tool in protein purification.
Johnston et al., U.S. Pat. No. 5,532,142, discloses recombinant vectors for overproducing plant virus proteinases in suitable hosts. Johnston et al. use the same purification protocol as that of Parks et al. (1995) to purify the 27 kDa NIa proteinase. The yield of purified proteinase is typically in the range of 5 mg/liter of cell culture and not all of it is active. Thus, the yield of active protein is very low.
As discussed above, the 27 kDa NIA proteinase contains an internal self-cleavage site that when cleaved, produces a proteinase with reduced substrate cleavage activity. At the present, there is no known method of stabilizing the proteinase, and there is no known method of obtaining large quantities of purified active 27 kDa NIa proteinase in large quantities. Accordingly, there is a need to develop a method of obtaining large quantities of purified active 27 kDa proteinase that will not cleave itself.